The goal of this work is to study mechanism-based strategies for enhancing the efficacy of immunotherapy against nicotine as a treatment for nicotine dependence in rats. Vaccination against nicotine has substantial efficacy in animal models for attenuating the behavioral effects of nicotine, and early clinical trials show increases in smoking cessation rates. While these data are encouraging, vaccines are not effective in all individuals. A major limitation of nicotine vaccines is their inability to consistently produce high serum antibody levels, and the large individual variability in these levels, which results in many non-responders. Passive immunization with nicotine-specific monoclonal antibodies, an alternative to vaccination, is highly effective in animals and can reliably produce high antibody levels, but is more costly than vaccination. Preliminary data in rats suggest that a combination of vaccination and passive immunization is more effective than vaccination alone. Aim 1 of this study will confirm and expand these studies using a target antibody concentration strategy (individualized supplementation of vaccinated rats with monoclonal antibody to achieve an effective serum antibody concentration) that could be applied clinically to exploit the advantages of both approaches while conserving monoclonal antibody. Aim 2 will examine whether the efficacy of immunotherapy can be enhanced by using a polyvalent nicotine vaccine. Preliminary studies have identified 2 nicotine immunogens that produce non cross-reacting antibodies and so are immunologically distinct. Their combined use is predicted to produce additive serum antibody levels and might also reduce the individual variability in response, resulting in fewer non-responders. This aim will evaluate the efficacy of 2 or 3 distinct nicotine immunogens combined into bi- or trivalent vaccines to determine whether they are more effective than a single immunogen. A second limitation of immunotherapy is that it likely blocks the direct effects of nicotine (reward and reinforcement) to a greater extent than subsequent effects that occur when nicotine is no longer present (craving). Aim 3 will examine the efficacy of immunotherapy combined with varenicline. Immunotherapy and varenicline have complementary but compatible mechanisms of action and effects, suggesting that their combination would be more effective than either one alone. For all 3 Aims, an integrated set of immunologic, pharmacokinetic and behavioral assays (nicotine self-administration, locomotor sensitization, discrimination) will be used to study mechanisms and assess efficacy. These Aims explore novel approaches to increasing nicotine vaccine efficacy using strategies with strong preliminary indications of usefulness and high potential for translation into clinical use. They address the principal limitations of a new therapeutic strategy which already appears to have clinical efficacy.